Typical building fire alarm systems include a number of fire detectors positioned through a building. Signals from those detectors are monitored by a system controller which, upon sensing an alarm condition, sounds audible alarms throughout the building. Flashing light strobes may also be positioned throughout the building to provide a visual alarm indication, with a number of audible alarms and strobes typically being connected between common power lines in a network. A first polarity DC voltage may be applied across those power lines in a supervisory mode of operation. In the supervisory mode, rectifiers at the alarm inputs are reverse biased so that the alarms are not energized, but current flows through the power lines so that the condition of those lines can be monitored. With an alarm condition, the polarity of the voltage applied across the power lines is reversed to energize all alarms on the network.
Typical strobes are xenon flash tubes which discharge very high voltages in the range of about 250 volts. Those high voltages are reached from a nominal 24 volt DC supply by charging a capacitor in increments with a rapid sequence of current pulses to the capacitor through a diode from an oscillator circuit. When the voltage from the capacitor reaches the level required by the flash tube, a very high voltage trigger pulse of between 4,000 and 10,000 volts is applied through a step-up transformer to a trigger coil about the flash tube. The trigger pulse causes the gas in the tube to ionize, drawing energy from the capacitor through the flash tube to create the light output.
Under the American Disability Act, and as specified in Underwriters Laboratories Standard UL 1971, the strobes must provide greater light intensity in order that the strobes can alone serve as a sufficient alarm indication to hearing impaired persons. Unfortunately, the strobes at the higher intensity levels have been reported to trigger epileptic seizures in some people.
In typical strobe systems, each strobe fires as the required firing voltage on the capacitor is reached. Since the strobes are free-running and tolerances dictate that the time constants of various strobes are not identical, the strobes appear to flash at random relative to each other. It is believed that a high apparent flash rate that results from the randomness of the high intensity strobes causes the epileptic seizures.
In accordance with the present invention, all strobes on a network are synchronized such that they all fire together at a predetermined safe frequency to avoid causing epileptic seizures. Additional timing lines for synchronizing the strobes are not required because the synchronizing signals are applied through the existing common power lines.
Accordingly, in a building alarm system having a plurality of warning strobes powered through common power lines, each strobe includes a flash lamp and a capacitor to be discharged through the flash lamp. A charging circuit powered by the common power lines applies a series of current pulses to the capacitor to charge the capacitor. The firing circuit responds to a change in voltage across the power lines to discharge the capacitor through the flash lamp.
In order to avoid overcharging of the capacitor as a strobe waits for the firing signal, each strobe further includes a voltage sensor for disabling the charging circuit when the capacitor reaches a firing voltage level.
In a preferred system, a network operates in a supervisory mode in which current flows from a system controller through the power lines to assure the integrity of the network during nonalarm conditions. Further, during an alarm condition, the system controller may code the synchronizing signals so that the timing of the flashing strobes indicates the location in the building at which the alarm condition was triggered.